The present invention relates to vehicle security systems, and more particularly to systems employing a siren with a backup rechargeable battery.
Presently sirens with a built-in rechargeable battery are commonly used with car alarms. The built-in battery is supposed to insure that the siren will sound if either the vehicle's 12 V battery is disconnected or if the siren is disconnected from the alarm system. In either case, the siren will sound continuously until the built-in battery charge is depleted or, if the owner is within earshot, he may turn the siren off with a key switch built into the siren.
The present art has several drawbacks. The first one is that the rechargeable battery draws its charged current from the vehicle's battery at a constant charging rate regardless of whether the engine is on or off. In addition, since the rechargeable battery has a voltage of 7.2 V while the vehicle's battery is at 12 V-15 V there is a continuous current discharge of typically 35-55 milliamps from the vehicle's battery even when the rechargeable battery is fully charged. That discharge will deplete the vehicle's own battery if the car is not driven for a time period of two weeks or more. Consequently, the owner may be unable to start the vehicle. What is more, if the voltage of the vehicle's battery drops below the rechargeable battery, it will cause the siren to sound until the rechargeable battery is also depleted causing in the meantime serious disturbances due to noise pollution. Paradoxically the very same rechargeable battery that was intended to improve the effectiveness and security provided by the alarm may in fact directly cause it to fail and also damage the vehicle's battery in the process.
Lastly, if the rechargeable battery is unable to hold a charge for any reason or if it malfunctions, presently there is no way for the owner of the vehicle to know of the problem.